In St. Thomas I did three excursions. I had snorkeling in the morning, then back on the ship for lunch. After lunch I went to the Butterfly Farm and the Sky Ride. After that I had just a little time to check out a few shops before back on the ship.
In Labadee, I had a tour in the a.m., then snorkeling, lunch, and zip line.
RC suggests you have an hour in between excursions. So it is doable.